Polyimide is commonly used for planarizing semiconductor devices and to provide electrical isolation between interconnected structures. A good example has been demonstrated by Horng et al. in US patent application 2011/0092005, which is incorporated herein by reference. A parallel addressed array of macro LEDs with improved reliability was achieved by using polyimide to fill the gap between the macro LEDs. Another common application is polyimide encapsulation of LEDs to improve the light extraction efficiency because of the increased refractive index such as described by Chen et al. in US patent application 2011/0024720, which is incorporated herein by reference. In contrast to the proposed present invention the polyimide used in these applications is transparent or what is known as having a high optical clarity.
This document is applicable to all LED arrays. For the description provided it is mainly concerned with the difficulties in manufacturing micro-LED structures which in this case will refer to any LED structure of less than 100 microns diameter. There are a number of known and existing problems with micro-LED arrays. For example, there is strong optical crosstalk between adjacent pixels in conventional LED arrays, resulting in the LED display having poor contrast. To form a matrix-addressable LED array, isolated LED mesa columns need to be formed by dry etch. The large height difference, the sloped mesa and the sharp edges formed in the trench cause undesirable light scattering (bars surrounding the pixels), and thus reduce the display contrast of the LED array.
A further known problem with LED arrays is a reliability issue associated with the isolation layer. Conventionally, the isolation layer of p-contacts from n-contacts of a matrix LED array is made from dielectric materials (e.g. silicon oxide or silicon nitride). For instance, Dawson et al. in US patent US2006/0110839 A1, which is incorporated herein by reference, uses SiO2 to isolate the mesa with sloped sidewall. Although the sloped mesa can alleviate the step coverage issue of the dielectric layer normally associated with standard techniques, the device made this y can still have reliability issues, resulting in undesirable electric crosstalk or shorting. This is mainly due to the fact that the thin dielectric layer cannot reliably cover the mesa with a large height, and it may be stripped off during subsequent process due to possible adhesion issues. All of these factors mean that the devices need to be carefully manufactured to ensure that the sidewall is suitably angled and that there are no contaminants (e.g. pin-holes) which could cause electrical crosstalk.
The present invention seeks to overcome these said disadvantages and problems.
It is an object of at least one aspect of the present invention to obviate or mitigate at least one or more of the aforementioned problems. It is a further object of at least one aspect of the present invention to provide improved micro-LED arrays with improved display contrast and reduced optical cross-talk.
It is a further object of at least one aspect of the present invention to provide an improved method for manufacturing micro-LED arrays with improved display contrast and reduced optical cross-talk.